Image diagnostic systems have been used for diagnosing arteriosclerosis, for preoperative diagnosis upon coronary intervention by a high-performance catheter such as a dilatation catheter (i.e., balloon catheter) or stent, and for assessing postoperative results.
Examples of these image diagnostic systems include intravascular ultrasound (IVUS) imaging systems. In general, the intravascular ultrasound imaging system is constructed to control an ultrasonic transducer to perform radial scanning within a blood vessel, to receive a reflected wave(s) (ultrasound echoes) reflected by biotissue (e.g. the blood vessel wall) by the same ultrasonic transducer, to subject the reflected waves to processing such as amplification and detection, and then to construct and display a tomographic image of the blood vessel on the basis of the intensities of the received ultrasound echoes.
In addition to these intravascular ultrasound imaging systems, optical coherence tomography (OCT) imaging systems have been developed in recent years for use as image diagnostic systems. In an OCT imaging system, a catheter with an optical fiber incorporated therein is inserted into a blood vessel. The distal end of the optical fiber is provided with an optical lens and an optical mirror. Light is emitted in the blood vessel while radially scanning the optical mirror arranged on the side of the distal end of the optical fiber, and based on light reflected from biotissue forming the blood vessel, a tomographic image of the blood vessel is then constructed and displayed.
Improved OCT imaging systems have been proposed in recent years which make use of a wavelength swept light source.
As described above, image diagnostic systems include several different kinds of systems which use different detection principles. Nonetheless, they are all characterized in that a tomographic image (i.e. cross-sectional image) is constructed and displayed by performing radial scanning with a probe. Accordingly, these image diagnostic systems each have merit in that the time required for a diagnosis by the system can be shortened by making radial scanning faster to increase the scanning speed in the axial direction.
A specific description will be made about an IVUS imaging system as an example. With the IVUS imaging system, tomographic images are generally extracted by performing radial scanning to transmit and receive ultrasounds while rotating an ultrasonic transducer at 1,800 rpm or so.
The setting at 1,800 rpm or so is attributed to the fact that the frame rate of video signals for successively outputting images to a display such as a CRT or LCD is 30 fps (30 frames/sec=1,800 frames/min) in many instances. Therefore, no additional tomographic image or images can be outputted to the side of the display even if the radial scanning is performed at a higher speed.
However, 1/30 second is needed to scan one frame of a tomographic image when the rotational speed of the ultrasonic transducer is 1,800 rpm or so. In this case, an image blur may occur by a heart beat while scanning one frame of the tomographic image. Faster radial scanning is hence desired.
As mentioned above, however, such faster radial scanning is still incapable of displaying acquired images in real time unless the frame rate of video signals for the display is made faster. Therefore, it may be possible to contemplate, for example, such a display method that without real-time construction of tomographic images, received signals are all converted into digital data by an A/D converter and once stored in a mass storage device such as a hard disk or semiconductor memory, and subsequent to completion of the data acquisition, all frames of the tomographic images are constructed from the recorded data and are then displayed.
For example, U.S. Pat. No. 6,315,722 discloses a method in which data produced based on received signals of ultrasounds are all stored in real time in a mass storage device such as a hard disk.
It is, however, impossible to confirm in real time whether or not data have been normally acquired, if the method disclosed in U.S. Pat. No. 6,315,722 is used to once store all the data without real-time construction and display of tomographic images and subsequent to the completion of the acquisition of the data, to construct and display all frames of tomographic images.
If data cannot be acquired for one reason or another, it thus becomes necessary to reinsert a catheter into the patient and to acquire data. This is certainly inconvenient. Further, the incapability of real-time confirmation of tomographic images as a result of the adoption of faster radial scanning by the probe is not well suited as an image diagnostic system.